PS. It is the purpuse that network x.x.x.x comes from AS1. This should
be allowed. The only mistake in the
topology is that someone forgot to remove the network statement on R3,
where this network was previously situated.

1) setting the noexport community on R2 will not prevent R2 from putting
the route in EIGRP. Also R3 would still
advertise the network to R2.

I have been thinking about some sort of filtering on R3 so that the
network x.x.x.x command will only
inject networks into BGP when a) they are in the routing table en b)
they are NOT tagged. I would tag
redistributed routes on R2...but i havent found a way to implement
this...

I believe that if you were to use the community attribute in BGP and set
it to No Export this would break the loop. Or, we could also create an
expression with an as-path list to only allow networks that originated
from the advertising AS. Does this work for you?

The network x.x.x.x enters AS2 through router R2, this router
redistributes the route into EIGRP, and now,
the network x.x.x.x is part of the routing table on router R3, so R3
suddenly starts advertising network x.x.x.x back to R2 over IBGP
R2 prefers this route (AS path is empty). R2 starts advertising network
x.x.x.x back out AS2 to R1 and stops redistributing x.x.x.x into EIGRP
(by default, no redistribution of IBGP routes). So the route is removed
from the routing table on R3, so that R3 stops advertising the route
again.
When R2 removes the route from BGP table, it prefers back the route
received from R1 and starts advertising the route again to R3 over
EIGRP....
and so we have a beautifull routing loop and lots of constant BGP
advertisements going on forever.....

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